Heterogeneous Dynamics of Multilayered Thin Polymer Films

Abstract

The glass transition and related dynamics of two types of multilayered thin films were investigated using differential scanning calorimetry and dielectric relaxation spectroscopy, to clarify the nature of heterogeneous dynamics in thin polymer films. First, the $\alpha$-process of multilayered thin films of poly(2-chlorostyrene) (P2CS) and polystyrene (PS) with various geometries was investigated during annealing process. The relaxation rate of the P2CS layer increases near the upper electrode with annealing, while that near the bottom electrode remains almost constant or slightly decreases with annealing. The relaxation strength for the $\alpha$-process of the P2CS layer increases with annealing near the upper electrode, while it decreases near the bottom electrode. A distinct positional dependence of the $\alpha$-process could be observed in the multilayered films. Second, the glass transition temperature, $T_\mathrm{g}$, and the dynamics of the $\alpha$- and $\beta$-processes for stacked thin films of poly(methyl methacrylate) (PMMA) were investigated during the annealing process. The $T_\mathrm{g}$ and the dynamics of the $\alpha$-process of as-stacked PMMA thin films exhibit thin-film-like behavior. Annealing at high temperature causes the $T_\mathrm{g}$ to increase from the reduced value, and causes the dynamics of the $\alpha$-process to become slower approaching those of the bulk. Contrary to the $\alpha$-process, the relaxation time of the $\beta$-process is almost equal to that of the bulk PMMA, and is unaffected by the annealing process. The fragility index increases with annealing, which suggests that the glassy state of the stacked thin films changes from strong to fragile.